Programmable voltage divider

ABSTRACT

A test circuit and programmable voltage divider that may be used in the test circuit. The programmable voltage divider develops a voltage difference signal that may be digitally selected. The test circuit may be used to test and characterize sense amplifiers. The programmable voltage divider develops a signal with a selected polarity and magnitude that is provided to a sense amplifier being tested. The sense amplifier is set and its output latched. The latch contents are checked against an expected value. The difference voltage may be changed and the path retested to find passing and failing points.

CROSS REFERENCE TO RELATED APPLICATION

The present invention is a divisional of U.S. patent application Ser.No. 11/028,174, now U.S. Pat. No. 7,447,964, “DIFFERENCE SIGNAL PATHTEST AND CHARACTERIZATION CIRCUIT” to Yuen H. Chan et al., filed Jan. 3,2005, assigned to the assignee of the present invention and incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention is related to small signal circuit design and moreparticularly to testing, characterizing and evaluating circuitsensitivity to a difference signal.

BACKGROUND DESCRIPTION

Integrated circuits (ICs) are commonly made in the well-knowncomplementary insulated gate field effect transistor (FET) technologyknown as CMOS. CMOS technology and chip manufacturing advances haveresulted in a steady decrease of chip feature size to increase on-chipcircuit switching frequency (circuit performance) and the number oftransistors (circuit density). In what is typically referred to asscaling, device or FET features are shrunk to shrink correspondingdevice minimum dimensions including both horizontal dimensions (e.g.,minimum channel length) and vertical dimensions, e.g., channel layerdepth, gate dielectric thickness, junction depths and etc. Shrinkingdevice size increases device density and device performance, as well asreduces device-operating conditions, i.e., chip and correspondingly,device supply voltages and voltage swings. Consequently, as a result ofscaling otherwise seemingly neglectable device-to-device variations(e.g., length, width, threshold and etc.) have caused serious designproblems, especially in signal critical circuits such as memory senseamplifiers.

A typical CMOS circuit includes paired complementary devices, i.e., ann-type FET (NFET) paired with a corresponding p-type FET (PFET), usuallygated by the same signal. Since the pair of devices have operatingcharacteristics that are, essentially, opposite each other, when onedevice (e.g., the NFET) is on and conducting (ideally modeled as aclosed switch), the other device (the PFET) is off, not conducting(ideally modeled as an open switch) and, vice versa. So, for example, aCMOS inverter is a series connected PFET and NFET pair that areconnected between a power supply voltage (Vdd) and ground (GND).

An ideal static random access memory (SRAM) cell includes a balancedpair of cross-coupled inverters storing a single data bit with a high atthe output of one inverter and a low at the output of the other. A pairof pass gates (also ideally, a balanced pair of FETs) selectivelyconnects the complementary outputs of the cross-coupled inverter to acorresponding complementary pair of bit lines. A word line connected tothe gates of the pass gate FETs selects the cell, connecting the cellcontents to the corresponding complementary pair of bit lines. During aread, each cell on the selected word line couples its contents to itscorresponding bit line pair through NFET pass gates. Since the bit linepair is typically pre-charged to some common voltage, initially, theinternal (to the cell) low voltage rises until one of the bit line pairsdroops sufficiently to develop a small difference signal (e.g., 30 mV).A simple ideal sense amplifier or, sense amp, is a matched pair ofcross-coupled common-source devices connected between a bit line pairand an enable source line. Device imbalances in matched cell devices orthe matched sense amp pair can unbalance the pair to seriously erode thesense signal margin and even cause sense amplifier errors.

Leakage currents can cause an inadequately balanced sense amplifier toself-trigger. Leakage from high floating-device body-voltages may causelarge offset voltages scattered unevenly in SOI devices that may triggerthe sense amplifier prematurely, latching false data. Similarly, SRAMcells can become instable from such leakage and cell performance maydegrade. Robust sensing techniques have been developed to subside todeal with device variability. However, evaluating such a sensingtechnique requires providing a variable differential signal that may beslewed within a range of interest. This may be done for an entire SRAMdata path (e.g., macro or chip), for example, by varying array/cellsupply voltage and determining read and write failing points.Unfortunately, this only gives an overall figure of merit for the datapath. Because circuits such as sense amps, of necessity, are verysensitive, it is not particularly helpful in evaluating such robustsensing circuits. It has been especially difficult to evaluate circuitresponse to a small voltage differential in such a sense circuitisolated from SRAM cells, i.e., outside of a data path. Consequently, itis difficult to characterize and evaluate state of the art SRAM cellsensing circuits.

Thus, there is a need for circuit that reliably test and characterizeSRAM cell sensing circuits and especially for simple and in-line testand characterization circuits that test small signal circuits to assistin deciding the merit of new sensing circuits.

SUMMARY OF THE INVENTION

It is a purpose of the invention to derive meaningful test results fromtesting isolated sense amplifiers;

It is another purpose of the invention to characterize and compare senseamplifier designs;

It is yet another purpose of the invention to programmably generate adifference signal;

It is yet another purpose of the invention to provide a programmabledifference signal to sense amplifiers for determining a sense amplifiersense point and characterizing the sense amp.

The present invention relates to a test circuit and programmable voltagedivider that may be used in the test circuit. The programmable voltagedivider develops a voltage difference signal that may be digitallyselected. The test circuit may be used to test and characterize senseamplifiers. The programmable voltage divider develops a signal with aselected polarity and magnitude that is provided to a sense amplifierbeing tested. The sense amplifier is set and its output latched. Thelatch contents are checked against an expected value. The differencevoltage may be changed and the path retested to find passing and failingpoints.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 shows a block diagram example of a preferred embodiment signalmargin characterization and test circuit;

FIG. 2 shows a timing example of a typical compare for the signal margincharacterization and test circuit, comparing data in as a zero and aone;

FIG. 3 shows an example a preferred programmable voltage divider;

FIG. 4 shows a timing example of a typical difference signal generationfor the voltage divider circuit;

FIG. 5 shows an example of a preferred comparator.

DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to the drawings and, more particularly, FIG. 1 shows a blockdiagram example of a preferred embodiment signal margin characterizationand test circuit 100. Preferably, the signal margin characterization andtest circuit 100 is formed in the complementary insulated gate fieldeffect transistor (FET) technology known as CMOS. A clock select circuit102 receives a global clock 104 and a clock select signal 106 and, whenselected, provides a local clock 108 to an input data latch 110. Theinput data latch 110 receives a data input value 112 (a “1” or a “0”)and provides a complementary output pair 110T, 110C as an input to apreferred voltage divider circuit 114, described in further detailhereinbelow. A 2:4 decoder 116 decodes a circuit test (e.g., sense amp)select 118 and provides a decoded sense amp select 120 to the preferredvoltage divider circuit 114. Voltage difference select signals 122 tothe preferred voltage divider circuit 114 select a voltage differencethat is provided on one pair of difference signal outputs 124-1-124-4,as selected by the decoded sense amp select 120. A sense enable circuit126 receives a sense select signal 128 and selectively gates the localclock 108 to generate a sense amp enable (sae) 130 and sense amp resetor reset enable (rse) 132. The difference signal outputs 124-1-124-4 areinputs to corresponding sense amplifiers 134-1-134-4. The sense ampenable 130 in combination with a corresponding one of decoded sense ampselect 120 selects one of sense amplifiers 134-1-134-4. Each senseamplifier 134-1-134-4 provides a complementary output to a correspondingoutput data latch 136-1-136-4. The output data latches 136-1-136-4provide a latched data output 138-1-138-4 to a comparator 140, alsodescribed in further detail hereinbelow. The comparator 140 compares thecontents of the output data latch 136-1-136-4 for the selected senseamplifier 134-1-134-4 against the contents of the input data latch 110(at 110T) and provides an indication of a match at output 142.

So, with the clock select signal 106 asserted, the global clock 104 ispassed to the local clock 108, clocking data 112 in the input data latch110 and selectively clocking the sense enable 126. The preferred voltagedivider circuit 114 receives the latched complementary data pair 110T,110C. In response to the reset enable 132 from sense enable 126, thepreferred voltage divider circuit 114 generates a difference signal withpolarity determined by data latch 110 contents. The magnitude of thedifference is selected by the voltage difference select signals 122. Thedifference signal is selectively passed out on a selected output124-1-124-4, selected by the decoded sense amp select 120. In responseto the sense amp enable 130 the selected sense amp 134-1-134-4 senses(or fails to sense) the difference on the selected 124-1-124-4. Thesensed value in the selected sense amp 134-1-134-4 is latched in arespective one of the output data latches 136-1-136-4. The sensedresults (from the output data latches 136-1-136-4) is compared againstthe expected result, i.e., from the input data latch 110. Thus, byshifting or stepping the voltage difference from the preferred voltagedivider circuit 114, the sense amplifiers 134-1-134-4 receive aselectively varied signal that characterizes the sense amp response,e.g., indicates sense margin and a minimum sense signal may bedetermined. It should be noted that although described in terms ofselecting one of 4 sense amps for test and/or characterization, thepresent invention has application to testing and characterizing anynumber of any type of circuit receiving a difference signal.

FIG. 2 shows an example of typical compare timing for the signal margincharacterization and test circuit 100 of the example of FIG. 1, for bothvalues of data—in 112, as a zero 144 and a one 146. In this example, theclock select signal 106 and the sense select signal 128 are heldasserted to pass the global clock 104 and local clock 108 through theclock select circuit 102 and sense enable circuit 126, respectively.Also in this example, both the sense amp enable 130 and reset enable 132are asserted low, i.e., the sense amplifiers 134-1-134-4 are enabled bya low on the sense amp enable 130 and reset by a low on the reset enable132. So, in this example, the voltage difference select signals 122 tothe preferred voltage divider circuit 114 are selected to develop asignal at the difference signal pair 124-4T, 124-4C that isapproximately 20 millivolts (20 mV), with 0.10V for the high and 0.08Vfor the low, respectively. Also in this example, the difference signalpair 124-4T, 124-4C recovers 148 relatively quickly.

FIG. 3 shows an example a preferred voltage divider circuit 114 in moredetail, which in this example is a programmable voltage divider. Adifference signal is developed in an active resistor network 1140 andselectively passed to a 1 of 4 select 1142. The complementary input datapair 110T, 110C are provided to inverters 1144, 1146. Inverters 1144,1146 gate a supply transistor 1148, 1150, a p-type (PFET) in thisexample. Parallel PFETs 1152, 1154-1, 1154-2, 1154-3, 1154-4, areconnected between the drains of supply PFETs 1148, 1150 at a switcheddifference signal pair 1156, 1158. The source of each supply PFET 1148,1150 is connected to a supply voltage. Parallel PFETs 1154-1, 1154-2,1154-3, 1154-4, are gated by a respective one of the voltage differenceselect signals 122-1, 122-2, 122-3, 122-4 and PFET 1152 is tied on,i.e., grounded gate. The remaining voltage difference select signals122-5, 122-6 each gate a pair of supply return transistors 1160-5,1162-5 and 1160-6, 1162-6, respectively. The sources of supply returntransistors 1160-5, 1160-6 are connected between ground and one of theswitched difference signal pair 1156. Similarly, supply returntransistors 1162-5, 1162-6 are connected between ground and the other ofthe switched difference signal pair 1158. The 1 of 4 select 1142includes 4 pair of pass gates (e.g., 1164T, 1164C) gated by a NAND gate1166. The reset enable 132 is an input to each NAND gate 1166, whichselectively passes a corresponding sense amp select signal 120-1-120-4to select one of the 4 pair of pass gates 1164T, 1164C. Each pair ofpass gates 1164T, 1166C selectively couples the difference voltage onthe switched difference signal pair 1156, 1158 to one of the differencesignal output pairs 124-1-124-4. A body contact may be provided to eachindividual PFET 1148, 1150, 1152, 1154-1, 1154-2, 1154-3, 1154-4,1160-5, 1162-5, 1160-6, 1162-6, 1164T, 1164C, to any combination thereof(e.g., shared contacts to one or more) or forgone completely, i.e., oneor more or all floating body PFETs as desired.

With all of the voltage difference select signals 122-1, 122-2, 122-3,122-4, 122-5 and 122-6 high, the active resistor network 1140 isswitched off. Since PFET 1152 is gated on, the switched differencesignal pair 1156, 1158 are effectively shorted together through 1152.So, regardless of the contents of input data latch 110, one of thecomplementary data pair 110T, 110C, one is low (e.g., 110C) and theother (110T) is high. In response to the low input, the output of thecorresponding inverter 1144 is high, turning off the respective PFET1148; and, in response to the high on the other input, the output ofcorresponding inverter 1146 is low, turning on the respective PFET 1150.However, since the switched difference signal pair 1156, 1158 areeffectively shorted together and there is no current path to ground,both are high. If one pair of pass gates 1164T, 1164C is selected, thehigh is passed on both output lines, e.g., 124-4T, 124-4C.

FIG. 4 shows a timing example of a typical difference signal generationfor the voltage divider circuit 114 of the example of FIG. 3. In thisexample, the data in is switched on each cycle as is reflected by thestate change of the complementary data pair 110T, 110C with each resetenable 132 cycle. As in the example of FIG. 2, sense amps are enabled(not shown in this figure) when the reset enable 132 is high. Thus, intime window 150 a difference signal develops on the selected both outputlines, e.g., 124-4T, 124-4C. Coupling noise is shown on the unselectedoutput pair 124-1T, 124-1C, which are both floating to allow theunselected sense amplifier (e.g., 134-1 in FIG. 1) to capacitivelycouple the reset enable signal 132 back onto the floating pair 124-1T,124-1C. However, since the respective sense amplifier is unselected,this noise is ignored.

However, if either or both of 122-5 and 122-6 are low, either or both ofsupply return transistors 1160-5, 1162-5 and 1160-6, 1162-6,respectively, provide a path to ground at both of the switcheddifference signal pair 1156, 1158. Thus, with either or both of 122-5and 122-6 low, the series connected PFETs act as a voltage divider. So,in the example with 110C low and 110T high, PFET 1150 and PFET 1162-5(and/or 1162-6) provide one path to ground; and, series connected PFET1152 and PFET 1160-5 (and/or 1160-6) provide a parallel partial path (toand through 1162-5 and/or 1162-6) to ground. Further, if any of thevoltage difference select signals 122-1, 122-2, 122-3, 122-4 are low,corresponding ones of parallel PFETs 1154-1, 1154-2, 1154-3, 1154-4 areon reducing the path resistance of PFET 1152 and correspondingly, thedifference signal. Thus, a difference voltage develops depending upondevices sizes of the on-PFETs as selected by the voltage differenceselect signals 122-1, 122-2, 122-3, 122-4, 122-5 and 122-6. Whateverdifference is selected, however, is passed to the selected pair of passgates 1164T, 1164C to the output lines, e.g., 124-4T, 124-4C.

FIG. 5 shows an example of a preferred comparator 140. The contents ofthe output data latch 136-1-136-4 are provided to a 4:1 encoder thatincludes 4 two input NAND gates 1400-1-1400-4 and four input NAND gate1402. Each of the 4 two input NAND gates 1400-1-1400-4 combines one theoutput of one data latch 136-1-136-4 with a corresponding sense ampselect signal 120-1-120-4. NAND gate 1402 combines the outputs of the 4two input NAND gates 1400-1-1400-4. The output 1404 of the 4:1 encoderfrom NAND gate 1402 is compared against the input data 110T in a comparecircuit 1406 that, in this example includes inverters 1408, 1410, andtwo tri-statable buffers 1412, 1414. Inverter 1408 inverts the encoderoutput 1404 and inverter 1410 inverts the input data 110T. Tri-statablebuffer 1412 is a non-inverting buffer and tri-statable buffer 1414 is aninverting totem driver buffer. So, when totem driver 1414 is driving theoutput 1416, PFET 1418 supplies totem power and inverter 1408 suppliestotem ground; otherwise, the totem driver 1414 is in its high impedancestate. The output 1416 of the compare 1406 is latched in a clockedoutput latch 1420, clocked by the local clock 108. An inverter 1422buffers the output 1416 of the compare 1406. The local clock 108 ispasses through a pair series connected inverters 1424, 1426 whichprovide complementary enable signals to non-inverting tri-statablebuffer 1428 at the output of inverter 1422. A pair of cross coupledinverters 1430, 1432 latch the other side of non-inverting tri-statablebuffer 1428. A pair of series inverters 1434, 1436 buffer the crosscoupled inverters 1430, 1432 and provide the compare output 142.

So, the outputs of the two input NAND gates 1400-1, . . . , 1400-4 arehigh unless the data output 138-1-138-4 from the selected output datalatch 136-1-136-4 is high, i.e., both the asserted sense amp selectsignal 120-1-120-4 and the corresponding data output 138-1-138-4 arehigh. If all of the outputs of the two input NAND gates 1400-1, . . . ,1400-4 are high, the output of NAND gate 1402 is low (indicating asensed zero) and the output of inverter 1408 is high. Non-invertingtri-statable buffer 1412 is on and totem driver 1414 is off (hi-Z). If,however, one output of a respective two input NAND gate 1400-1, . . . ,1400-4 is low, because the asserted sense amp select signal 120-1-120-4and the corresponding data output 138-1-138-4 are both high, then theoutput of NAND gate 1402 is high (indicating a sensed one) and theoutput of inverter 1408 is low. Non-inverting tri-statable buffer 1412is off (hi-Z) and totem driver 1414 is on.

If the input data 110T matches the output of NAND gate 1402, theselected sense amp has sensed the difference signal correctly. So, if aone is sensed correctly, the output of inverter 1410 is low. The totemdriver 1414 inverts the low so that the compare output 1416 is high. Ifa zero is sensed correctly, the output of inverter 1410 is high. Thenon-inverting tri-statable buffer 1412 passes the high so that, again,the compare output 1416 is high. However, if a one is sensedincorrectly, the output of inverter 1410 is high and the totem driver1414 inverts the high, providing a low at the compare output 1416.Likewise, if a zero is sensed incorrectly, the output of inverter 1410is low and the non-inverting tri-statable buffer 1412 passes the low tothe compare output 1416.

Buffer inverter 1422 inverts the compare output 1416. The invertedcompare signal passes through non-inverting tri-statable buffer 1428when the local clock 108 is low and remains latched in cross coupledinverters 1430, 1432 when the clock rises. The inverting latch outputfrom inverter 1430 re-inverts the compare results. The re-invertedresults pass through the pair of series inverters 1434, 1436, emergingas a match indication at the compare output 142.

Advantageously, a preferred embodiment circuit develops difference(analog) signals that may be used independent of SRAM cells as testsignals to test and characterize circuits, e.g., sense amplifiers. Thetest signals may be generated and controlled digitally; the circuitunder test (e.g., a sense amp) tested; and test results reported in adigital output. Further, the same difference signal may be applied tovarious sense amplifier topologies or designs for a realistic in situcircuit comparison.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims. It is intended that all such variations andmodifications fall within the scope of the appended claims. Examples anddrawings are, accordingly, to be regarded as illustrative rather thanrestrictive.

1. A programmable voltage divider comprising: a complementary data inputselecting a difference signal polarity; a programmable resistordeveloping a difference voltage having the selected said differencesignal polarity, wherein said programmable resistor comprises a p-typefield effect transistor (PFET) resistor network, said programmableresistor further comprising: a pair of inverters each receiving one saidcomplementary data input, a pair of supply PFETs mutually exclusivelysupplying power to said PFET resistor network, each connected between asupply and one of a pair of difference signal lines, an output of eachof said pair of inverters being connected to a gate of a correspondingone of said pair of supply PFETs; and a selectable output selectivelyproviding a difference voltage as a difference signal output.
 2. Aprogrammable voltage divider as in claim 1, wherein said programmableresistor further comprises a plurality of selectively-gatedparallel-connected said PFETs connected between said pair of differencesignal lines.
 3. A programmable voltage divider as in claim 2, whereinsaid programmable resistor further comprises a plurality ofselectively-gated series-connected supply return said PFETs connectedbetween a supply return and one of said pair of difference signal lines.4. A programmable voltage divider as in claim 3, wherein saidprogrammable resistor further comprises a tied-on PFET in parallel withsaid selectively-gated parallel-connected PFETs; and, wherein each ofsaid selectively-gated parallel-connected PFETs and each pair of saidselectively-gated series-connected supply return PFETs are connected toa corresponding one of a plurality of resistance programming selects, afirst PFET of said each pair being connected between one of said pair ofdifference signal lines and a supply return, and a second PFET of saideach pair being connected between the other of said pair of differencesignal lines and said supply return.
 5. A programmable voltage divideras in claim 4, wherein said selectable output select comprises aplurality of pairs of pass gates, each connected between said pair ofdifference signal lines and a corresponding difference signal output. 6.A programmable voltage divider as in claim 5, wherein said pass gatesare PFETs.
 7. A programmable voltage divider as in claim 6, wherein oneor more PFETs are floating body PFETs.